To produce lower cost solar cells to facilitate large scale electrical applications of solar electricity, it is important to provide lower cost substrate materials for making the solar cells. A known method for achieving this objective is to grow a crystalline silicon using a continuous ribbon growth process as described in U.S. Pat. Nos. 4,661,200; 4,627,887; 4,689,109; and 4,594,229.
According to the continuous ribbon growth method, two high temperature material strings are introduced through holes in a crucible which contains a shallow layer of molten silicon. A crystalline silicon ribbon forms as the melt solidifies while being pulled vertically from the melt. The strings stabilize the position of edges of the growing ribbon. The molten silicon freezes into a solid ribbon just above the layer of molten silicon. To make this ribbon silicon process continuous, more silicon is added to the melt as the crystalline silicon is formed to keep the amount of melt constant. Keeping the amount of the melt constant during the growth process is important in order to achieve uniform and controllable growth of the crystalline silicon. It is also important to keep the thermal environment of the cooling ribbon constant. Slight changes in the depth of the melt and consequent changes in the vertical position of the solid-liquid interface can significantly change this thermal environment. For example, it has been found that variations in the melt depth of more than about one millimeter can result in a markedly different thickness and introduce a residual stress state of the grown silicon ribbon. For all of these reasons, a constant melt level is an important element in insuring uniform, controlled growth of silicon ribbon.
One way to control the depth of the melt is to continuously measure the depth of the melt and to control the rate at which silicon is added to the melt based on the measured depth. The depth of the melt must be measured in a way that is simple, cost effective, accurate, non-contaminating, and capable of being connected in a feedback loop to the feeder.
Several methods for measuring the melt depth are known. For example, one method uses an oscillating probe consisting of a thin graphite rod, with the making or breaking of electrical continuity as the probe touches the molten silicon giving the vertical position of the melt surface. This requires an actuator mechanism, a position sensor, and a clear vertical access to the melt. This method, however, presents a problem because one implementation of the growth technique relies on precisely positioned insulating shields to shape the cooling profile of the ribbon. Allowing room for a mechanical linkage is an undesirable constraint in the design and placement of these insulating shields.
Another known method utilizes a laser beam which is reflected from the melt surface. This technique is used in metal foundry work for melt depth measurement. The problem with this method, however, is that providing a clear optical path to the melt surface hampers the design of insulating components. In addition, it is difficult to maintain clean viewing ports in an environment where vapor deposited silicon oxide is common.
A third method is more indirect and involves measuring the weight and size of the grown crystal to track the loss of silicon from the crucible. This technique is described in U.S. Pat. No. 4,936,947. A major disadvantage of this method, aside from its complexity, is that it is applicable for growth of discrete crystals in a batch mode and is less applicable for continuous growth.
The above described methods are inadequate in satisfying the necessary criteria for making cost-effective solar cell substrate material. It is therefore an object of the present invention to provide a method which does satisfy these criteria.